These investigations are concerned with causal systems \((W,E)\)  (), which describe the localizability of relativistic quantum systems following Wightman [1]}. What is going beyond the physical ideas developed by Newton, Wigner [2]} and Wightman [1]} is the inclusion of special relativity and causality. So the representation \(W\)  of the Poincaré group \(\tilde{\mathcal {P}}\)  describing the relativistic quantum system under consideration and the projection valued measure (PM) \(E\)  describing the localization of the system on every spacelike hyperplane of Minkowski space satisfy Poincaré covariance (i) and the causality condition (ii), namely
